1. Field of the Invention
The present invention relates to a method and apparatus for controlling a printer head of e.g. an LED printer, and more particularly to a method and apparatus of the above type for use in halftone printing.
2. Description of Related Arts
In conventional optical printers such as an LED printer, such systems as the dither system and the density pattern system have been used for expression of gradation.
With these systems; however, while some improvement is possible in the gradation expression performance of the printer by increasing the matrix size, the increased matrix size inevitably invites substantial deterioration in image resolution.
In view of the above problem, the prior art has suggested gradation expression through multi-level dot control scheme.
For instance, a Japanese patent laid open under 59-127468 discloses such multi-level dot control scheme in which supply current values to respective LED elements are varied separately in correspondence with a plurality of predetermined dot gradation levels thereby to independently control the output light amounts of the respective LED elements within one scanning period.
A further Japanese patent laid open under 60-005387 (corresponding to U.S. Pat. No. 4,596,995) discloses an alternate multi-level dot control scheme in which driving times of the respective LED elements are separately varied in correspondence with a plurality of predetermined gradation levels thereby to independently control the per-scanning-period output light amounts of the respective LED elements.
In either of the above-described prior systems, the LED element is energized for light emission one time in one scanning period. Accordingly, in order to provide each LED element with a different current or a different driving time, a complicated circuit is needed for the discrete control of the currents and driving times for the elements. That is to say, with the above-described methods, it is impossible to utilize the conventional printer head or drive circuitry based on the binary-level control scheme.